The present invention relates to a coordinate graphic input tablet or more particularly, to a see-through coordinate graphic input tablet of the electrode-contact type.
Among various types of coordinate input tablets, see-throughness is required in those used as installed on or in front of a display unit such as CRTs, LCDs and the like, multiple-term input panels and graphic illustrations such as figures and letter signs. Several types of such see-through coordinate input tablets are hitherto proposed including those of the electrode-contact type by constructing a membrane switch using a transparent and electroconductive material, those of the optical-matrix type in which a light source and a photosensor are combined to detect the coordinates of a body on the path of the light beam, those of the pressure type in which the pressure produced by touching at a position on the tablet is detected, those of the electrostatic-capacity type in which the change in the electrostatic capacity produced by touching is detected, those of the surface acoustic wave type in which the coordinates of a point are detected by means of the time taken for the propagation of the surface acoustic waves, and so on.
The above named various types of the coordinate input tablets, however, have their respective problems and disadvantages. In particular, those other than the electrode-contact type are almost unusable practically in applications for high-speed input works continued for a length of time for the coordinates of a multiplicity of points such as on figures and letters because the resolving power of input therein is poor or can be improved only by use of a very elaborate and consequently very expensive peripheral circuits or by increasing the size of the equipments.
On the other hand, those of the first mentioned electrode-contact type are practically advantageous in respect of the relatively simple structure and the low costs for the peripheral circuits although some problems are left to be solved as described below.
The coordinate input tablets used for inputting figures, letters and the like are classified into digital ones and analogue ones. Namely, the digital-type coordinate input tablet is constructed by two tablet elements each made of an insulating substrate sheet provided on one surface with an electroconductive layer formed in a pattern of an array of parallel lines by the technique of etching and the two elements are disposed in parallel to each other to have the surfaces provided with the conductive layers facing with each other keeping a narrow space therebetween in such a manner that the directions of the line-wise patterns of the electroconductive layers on the two elements are perpendicular to each other. The coordinates of a point at which one of the elements is pushed can be detected by the combination of the conductive lines on the elements which have been brought into contact with each other by pushing. The coordinate input tablet of the analogue type is constructed by disposing a resistance element having, on the whole surface of an insulating substrate sheet, a uniform thin resistance film and an electrode element of also sheet-like form to serve as a common electrode in parallel keeping a narrow space therebetween so that the coordinates of a point at which either of the sheets is pushed to bring the two sheets in contact with each other can be detected by means of the resistance of the resistance element between the peripheries of the resistance element and the pushed position.
Regardless of the type which may be digital or analogue, the electrode-contact type coordinate input tablet has an element made by providing an insulating sheet such as a plastic film with an electroconductive thin layer. Such a layer is usually formed by the techniques of vapor deposition or sputtering with a metallic material such as silver, palladium and the like or a semiconductive metal oxide such as indium oxide In.sub.2 O.sub.3. Accordingly, the problems in the digital type tablets include the high cost to form the parallel line-wise pattern by the technique of etching and the difficulty in making electric connections between the lines and outer circuits in addition to the relatively high electric resistance of each of the lines, especially, when a high light-transmission is desired of the element necessitating to use extremely fine conductive lines or wires. In the analogue type tablets, on the other hand, it is an extremely difficulty matter to ensure high uniformity in the surface resistivity over the whole area of the resistance film so that the accuracy of the device is badly affected. The problems common to both of the digital and analogue type tablets are that high light transmission can hardly be obtained and the electroconductive layer is susceptible to mechanical damages due to the extremely small thickness to limit the yield of acceptable products and serviceable life thereof.